Self-controlling safety type LMFBR reactor disposed in deep ground layer

An entire reactor core is tightly sealed with mantles except for inlet and exit for non-radioactive secondary coolants. Further, the entire mantle-sealed structure is berried in the layer deeply underground. The sealed reactor core has a long tightly closed tubular container disposed at the lower portion thereof with a liquid evaporizing portion having liquid metal primary coolants sealed therein and a plurality of heat pipes at a gas condensing portion to the upper portion thereof respectively. Heat exchanging secondary coolants are caused to flow on the outside of the condensing portion in the upper portion of the heat pipes to conduct heat exchange between the heat pipe condensing portion and the secondary coolants. At least two fuel elements are stacked in the longitudinal direction of the heat pipes and shape memory alloys are put between them in the liquid metal primary coolants in the lower evaporizing portion of the heat pipes or at the outer circumference of the evaporizing portion for the lower portion of each of the heat pipes. Thus, the distance between each of the fuel elements is expanded or contracted depending on temperature change thereby enabling to control nuclear reactions automatically, save facilities and facilitate waste disposal. (K.M.)